This invention relates to a liquid level sensing apparatus and system for use in a wide variety of equipment requiring the monitoring of liquid tanks or containers.
The concept of monitoring liquid in a container by use of thermocouple junctions measuring temperatures at vertically spaced locations to recognize the presence of liquid by heat exchange therewith is already well known as disclosed in U.S. Pat. No. 3,280,627 to Cousins. According to the Cousins patent, all of the temperature sensing thermocouple junctions are interconnected in series and are equally heated by an electrical heating element within a tubular probe to provide a single combined voltage output from the series connected thermocouple junctions. The output of the thermocouple junctions will vary with the level of liquid into which the probe is inserted since each thermocouple junction generates a voltage dependent on localized heat exchange between the liquid body and the internally heated probe.
The foregoing concept, taught in the Cousins patent, of internally heating a probe to generate an output voltage at a thermocouple junction for level sensing purposes is also embodied in a power monitoring probe for a nuclear reactor according to U.S. Pat. No. 4,418,035 to Smith. Each thermocouple junction according to the Smith patent, however, is one of two such junctions forming a differential temperature type of sensor designed to also measure localized power output of the nuclear reactor as a function of the gamma ray heating of the body of the probe embedding the sensors therein as more fully explained in U.S. Pat. No. 4,298,430 to Rolstad et al. Thus, both the Smith and the Rolstad et al patents refer to the use of a plurality of separate sensor cables to respectively monitor local conditions (whether it be power rate or coolant level) at vertically spaced measurement zones. The Rolstad et al patent is made of record in the Smith patent, both patents being owned in common with the present application by the same assignee.
It will be apparent that the use of double junction sensors is of significance in regard to the localized nature of its temperature monitoring function so that separate signal outputs are required for the measurement zones. Although two sets of differential temperature junctions are interconnected in series according to U.S. Pat. No. 4,439,396 to Rolstad, also owned in common with the present application, the two sets of junctions are associated with a single measurement zone to improve signal strength and reduce signal error with respect to the signal output from each single measurement zone. However, the use of multijunction sensors with separate signal cables for each measurement zone had not been deemed to be advantageous for solely monitoring liquid level in a variety of different installations which sometimes require a relatively large number of vertically spaced measurement zones and provide severe spatial restrictions for the probe assembly.
Although single junction sensors interconnected in series, as disclosed in the Cousins patent aforementioned, are more suitable because of the associated single output cable arrangment, such level sensing arrangement is subject to signal error because of varying heat flow and temperature conditions, of the liquid between measurement zones. Such signal errors are not capable of being reliably corrected by acumulated error data because of the often unpredictable nature of heat flow conditions within the liquid body being monitored.
It is therefore an important object of the present invention to provide an improved temperature sensing type of liquid level monitoring probe which has the spatial advantages of a single signal cable type of probe without the aforementioned signal error disadvantages.